The microstrip technique is a planar technique used to produce lines conveying signals and antennas coupling such lines and radiated waves. It uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate separating them from a conductive layer on the bottom surface of the substrate and constituting a ground for the line or antenna. A patch is typically wider than a strip and its shape and dimensions are important features of the antenna. The substrate is typically a plane rectangular sheet of constant thickness and the patch is also typically rectangular. This is not obligatory, however. In particular, varying the thickness of the substrate, for example exponentially, widens the bandwidth of an antenna of the above kind, and the shape of the patch can in particular be circular. The electric field lines extend between the strip or patch and the ground layer through the substrate.
The technique differs from various other techniques that also employ conductive elements on a thin substrate, in particular the coplanar line technique in which the electric field is established on the top surface of the substrate and symmetrically between a central conductive strip and two circular areas on respective opposite sides of the strip, from which they are separated by respective slots. In the case of an antenna, a patch is surrounded by a continuous conductive area from which it is separated by a slot.
Antennas using the above techniques are typically, although not exclusively, resonant structures which are the site of standing waves enabling coupling with radiated waves.
A broad distinction can be drawn between diverse types of resonant structure that can be made using the microstrip technique and which correspond to respective modes of resonance of the structure. A first type is the commonest one and might be called the "half-wave" type. Taking one dimension of the patch as its length, in a direction called the longitudinal direction, the length is typically substantially equal to one half-wave, i.e. to half the wavelength of an electromagnetic wave propagating in that direction in the line comprising the ground, the substrate and the patch. The antenna is then called a "half-wave" antenna. That type of resonance can be generally defined by the presence of an electric current node at each of the two ends of the length, which can therefore also be equal to said half-wave multiplied by an integer other than 1. This number is typically an odd number. Coupling with radiated waves occurs at the ends of the length, in regions where the amplitude of the electric field in the substrate is maximum.
A second type of resonant structure that can be produced using the same technique might be called the "quarter-wave" type. It differs from the half-wave type in that the patch typically has a length substantially equal to one-quarter wave, i.e. to one-fourth of a wavelength, the length of the patch and the wavelength being defined as above. In this case the antenna is called a "quarter-wave" antenna. It is also different in that there is a clear short-circuit at one end of the length between the ground and the patch in order to impose a "quarter-wave" resonance mode. This type of resonance can be generally defined by the presence of an electric field node fixed by the short-circuit at one end of the length of the patch and by an electric current node at the other end of the length. The length can therefore be equal to an integer number of half-waves added to said quarter-wave. Coupling with the radiated waves occurs at the other end of the length, in the region in which the amplitude of the electric field through the substrate is maximum.
Other resonance modes can be established in planar antennas. They depend in particular on:
the configuration of the patches, which can incorporate slots, possibly radiating slots, PA1 the possible presence and location of short-circuits and electric models representative of short-circuits, which are not always equivalent, even approximately, to perfect short-circuits, for which the impedance would be zero, and PA1 the possible presence and location of coupling devices in the antennas for coupling their resonant structures to a signal processing unit such as a transmitter. PA1 it must be a dual frequency antenna, i.e. it must be capable of transmitting and/or receiving radiated waves efficiently on two widely spaced apart frequencies, PA1 it must be possible to connect it to a signal processor unit using a single connection line for all operating frequencies of a radiocommunication device, without giving rise to unwanted standing wave ratios on the line, and PA1 it must not be necessary to use a frequency multiplexer or demultiplexer to achieve this. PA1 Its implementation is complicated by the need to provide impedance converter means. PA1 It is difficult to adjust the resonant frequencies accurately to required values. PA1 Its overall size corresponds to three half-waves. PA1 Incorporating localized short-circuits complicates the implementation of the antenna. PA1 The coupling device of the antenna, which is in the form of a coaxial line, requires accurate adjustment of the position of the coaxial structure passing through the substrate to obtain good matching to a feed line having an impedance of 50 ohms for the two operating frequencies. PA1 The difference between the two resonant frequencies is too small in some applications. PA1 The need to use a matching system complicates the implementation of the antenna. PA1 The same can apply to implementing the coupling device of the antenna in the form of a coaxial line. PA1 enabling simple implementation of a dual frequency antenna, more particularly an antenna for which the ratio is in the range approximately 0.2 to approximately 0.8 and in particular around 0.5, by choosing more freely than previously the ratio between two wanted resonant frequencies of the antenna, PA1 giving the antenna a sufficiently wide bandwidth around each of the two resonant frequencies for a transmit frequency and a receive frequency to be situated within that bandwidth without crosstalk occurring, PA1 enabling easy and accurate adjustment of the two resonant frequencies, PA1 enabling use of a single coupling device whose impedance can easily be matched for each of the two resonant frequencies, and PA1 limiting the dimensions of the antenna. PA1 a signal processor unit adapted to be tuned to a frequency near to at least two predetermined frequencies to transmit and/or receive an electric signal at each of those two frequencies, and PA1 an antenna connected to the processor unit to couple said electric signal to radiated waves. The antenna is a microstrip antenna. Its patch has a rear edge provided with a short-circuit. It also has a front edge opposite the rear edge and two lateral edges joining the rear edge to the front edge. The short-circuit enables a quarter-wave resonance to be established in the antenna with an electric field node fixed by the short-circuit and a resonance path extending between said rear edge and said front edge. Said resonance is at a frequency which is one of said predetermined frequencies and constitutes a quarter-wave resonant frequency. In the device, the other predetermined frequency is a half-wave resonant frequency consisting of the frequency of a half-wave resonance established in the antenna with a resonance path extending between said two lateral edges. The types of resonance considered in the context of the invention are generally defined hereinabove.
There may be a plurality of resonance modes for a given antenna configuration, enabling the antenna to be used at a plurality of frequencies corresponding to those modes.
The present invention is particularly characterized by choosing certain "resonance paths", as explained hereinafter. The meaning of the expression "resonance path" as used hereinafter will now be defined:
Each resonance mode can be described as the result of superposing two waves propagating in opposite directions on the same path and reflected at the two ends of the path alternately. The path is imposed by the components of the antenna. It constitutes the "resonance path" for this resonance mode. It is rectilinear and longitudinal in the case of the half-wave and quarter-wave antennas previously mentioned. However, it can also be a curved radiating slot. In all cases the resonant frequency is inversely proportional to the time for which a traveling wave travels along the resonance path (see above). The expression "resonance mode" is sometimes replaced below by the term "resonance".
An antenna is typically coupled to a single processor unit such as a transmitter by a connection system including a coupling device incorporated in the antenna and a connection line external to the antenna connecting the coupling device to the signal processor unit.
In the case of a resonant structure transmit antenna, the respective functions of the coupling device, the connection line and the antenna are as follows: the function of the connection line is to convey a radio frequency or microwave frequency signal from the transmitter to the terminals of the antenna. The signal propagates along the whole of a line of this kind in the form of a traveling wave without its characteristics being significantly modified, at least in theory. The function of the coupling device is to convert the signal supplied by the connection line into a form in which it excites a resonance of the antenna, i.e. so that the energy of the traveling wave conveying the signal is transferred to a standing wave established in the antenna with characteristics defined by the antenna. Transfer is generally imperfect, i.e. the coupling device reflects some of the energy towards the connection line, which causes an unwanted standing wave in the line. The corresponding standing wave ratio varies as a function of frequency and the diagram of that variation defines the bandwidth(s) of the antenna. The antenna transfers energy from the standing wave to a wave radiated in space. The signal supplied by the transmitter is transformed a first time from a traveling wave to a standing wave and a second time into a radiated wave. In the case of a receive antenna, the signal takes the same forms in the same units but in the reverse order.
The coupling device and the connection line can be implemented using a technique other than the microstrip technique, for example in the form of coaxial or coplanar lines. To limit unwanted reflections their nature and dimensions are chosen to match the impedance of the various units through which the signals travel.
A transmit antenna connecting system is often referred to as an antenna feed line.
The present invention concerns antennas which can be included in various types of device, including mobile telephones, base transceiver stations for mobile telephones, motor vehicles, aircraft and airborne missiles. In the case of a mobile telephone the continuous nature of the bottom ground layer of a microstrip antenna provides an easy way to limit the amount of radiated power intercepted by the body of the user of the device. In motor vehicles and above all in aircraft or missiles which have a metallic outer surface and a curved profile for low aerodynamic drag, the antenna can be conformed to the profile so that it does not cause unwanted additional aerodynamic drag.
The invention is more particularly concerned with the situation in which an antenna of the above kind is required to have the following qualities:
The prior art describes or proposes many dual frequency microstrip antennas. They differ in terms of the means employed to obtain a plurality of resonant frequencies. Three such antennas will now be considered:
A first prior art antenna is described in U.S. Pat. No. 4,766,440 (Gegan). The patch 10 of the antenna is generally rectangular and so the antenna has two half-wave resonances whose paths are along a length and a width of the patch. It also has a U-shaped curved slot which is entirely inside the patch. The slot is a radiating slot and produces an additional resonance mode on a different path. By appropriately choosing its shape and dimensions, the frequencies of the resonance modes are tuned to required values, which leads to the possibility of transmitting a circularly polarized wave by associating two modes having the same frequency and crossed linear polarizations. The coupling device is in the form of a microstrip line which is also coplanar in the sense that the microstrip is in the plane of the patch and penetrates between two notches in it. The device includes impedance converting means for matching it to the various input impedances of the line at the various resonant frequencies used as operating frequencies.
The first prior art antenna has the following drawbacks:
A second prior art antenna differs from the previous one in that it uses a single resonance path. It is described in U.S. Pat. No. 4,771,291 (LO et al.). Its patch incorporates localized short-circuits and slots extending along respective straight line segments within the patch. The slots and short-circuits reduce the difference between two frequencies corresponding to two resonances which share the path but have two different modes, respectively designated by the numbers (0, 1) and (0, 3), meaning that the common path is occupied by one half-wave or three half-waves, depending on the mode concerned. In this way the ratio between the two frequencies can be reduced from 3 to 1.8. The localized short-circuits are provided by conductors extending through the substrate.
The second prior art antenna has the following drawbacks:
A third prior art dual frequency antenna differs from the previous two in that it uses a quarter-wave resonance. It is described in IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM DIGEST, NEWPORT BEACH, JUN. 18-23, 1995, pages 2124-2127 Boag et al. "Dual Band Cavity-Backed Quarter-wave Patch Antenna". A first resonance frequency is defined by the dimensions and the characteristics of the substrate and patch of the antenna. A resonance of substantially the same type is obtained on the same resonance path at a second frequency using a matching system.
The third prior art antenna has the following drawbacks: